Carbon wire and nano structure formed of carbon film and method of producing the same

ABSTRACT

There are provided a carbon wire using CNT or a similar carbon filament having a sufficiently low electrical resistance value, and a wire assembly employing that carbon wire. A carbon wire ( 1 ) includes an assembly portion ( 3 ) and a graphite layer ( 4 ). The assembly portion ( 3 ) is configured of a plurality of carbon filaments implemented as carbon nanotubes ( 2 ) in contact with one another. The graphite layer ( 4 ) is provided at an outer circumference of the assembly portion ( 3 ).

TECHNICAL FIELD

The present invention relates generally to carbon wires, wire assembliesand methods of producing the same, and particularly to carbon wires,wire assemblies and methods of producing the same, that employ aplurality of carbon filaments.

The present invention also relates to electrically conductive film,electrically conductive substrates, and transparent, electricallyconductive sheet including graphite film, that are obtained by exposinga carbon nanotube network to Ga vapor, and methods of producing thesame.

The present invention also relates to methods of obtaining graphite filmby exposing a carbon source to Ga vapor.

BACKGROUND ART

As one example of a carbon filament, a carbon nanotube (CNT) hasexcellent properties and is accordingly expected to be employed in avariety of industrial applications. For example, a CNT providessubstantially as low an electrical resistance value as copper and isthus considered to be used as a material for wire. Furthermore, such.CNT is produced in a variety of methods, as proposed for example inJapanese Patent Laying-Open No. 2007-112662 (Patent Document 1).

Japanese Patent Laying-Open No. 2007-112662 proposes a method in which ametal catalyst of gallium (Ga) is introduced in an amorphous carbon wirestructure and a direct current is applied thereto to produce a CNTsized, shaped and oriented as desired.

When a carbon atom is chemically bonded by an sp2 hybridized orbital, itforms a lattice-structured film having two dimensionally spreadcarbocyclic six-membered rings packed in a plane. This carbon atom's twodimensional planar structure is referred to as graphene. As a specialexample, graphene in a tubular closed structure is a carbon nanotube,and graphene layers stacked in a direction of a normal thereto aregraphite.

A carbon nanotube is a tubular material having a diameter equal to orsmaller than 1 μm, and ideally, a film in a lattice structure ofcarbocyclic six-membered rings has planes parallel to a tube's axis toform the tube, and a multiple of such tubes may be provided. The carbonnanotube is theoretically expected to exhibit a metallic property or asemiconducting property depending on how the lattice structured filmshave carbocyclic six-membered rings linked and the tube's thickness, andit is thus expected as a future high-performance material.

For example, Japanese Patent Laying-Open No. 2007-63051 (Patent Document2), Japanese Patent Laying-Open No. 2002-255528 (Patent Document 3),Japanese Patent Laying-Open No. 2003-238126 (Patent Document 4), andJapanese Patent Laying-Open No. 2000-86219 (Patent Document 5) discloseorganizing carbon nanotubes to provide a structure by dispersing carbonnanotubes in a dispersion medium for example ultrasonically to prepare adispersion liquid of carbon nanotubes which is in turn dropped on aplanar substrate and dried thereon to provide a thin film of carbonnanotubes. However, the thin film of carbon nanotubes has the carbonnanotubes interconnected simply by contacting one another, and is thusdisadvantageously high in contact resistance.

Graphite has a variety of electrical properties, as observed on graphitefilm, such as a bandgap, a fractional quantum Hall effect and the likevarying with in what size it is cut out, and is thus gaining a largeattention in recent years not only for physical phenomena but also interms of application to devices in the future.

K. S. Novoselov et. al., Science 306 (2004) pp. 666-669. (Non PatentDocument 1), K. S. Novoselov et. al., Proc. Natl. Acad. Sci. U.S.A. 102(2005) pp. 10451-10453. (Non Patent Document 2), C. Berger et. al., J.Phys. Chem. B108 (2004) pp. 19912-19916. (Non Patent Document 3), andYuanbo Zhang et. al., Nature 438, pp. 201-204 (10 Nov., 2005) (NonPatent Document 4) disclose techniques used to produce a monolayer ofgraphite film, i.e., graphene.

Typical conventional techniques are provided by K. S. Novoselov et. al.,Science 306 (2004) pp. 666-669. (Non Patent Document 1), and K. S.Novoselov et. al., Proc. Natl. Acad. Sci. U.S.A. 102 (2005) pp.10451-10453. (Non Patent Document 2). More specifically, Scotch tape isstuck on graphite crystal to peel off graphite to leave a single sheetof graphene on a silicon substrate having a surface oxidized and amonolayer of graphene is found and utilized. This technique is a ratherprimitive technique.

C. Berger et. al., J. Phys. Chem. B108 (2004) pp. 19912-19916. (NonPatent Document 3) discloses that a high temperature process at1400-1600° C. is performed in an environment of ultrahigh vacuum todecompose a SiC monocrystalline surface and after Si is selectivelysublimated a monolayer of graphene is synthesized. Furthermore, it isalso disclosed that a diamond microcrystal is first formed and thenprocessed at 1600° C. to obtain graphene from diamond.

Yuanbo Zhang et. al., Nature 438, pp. 201-204 (10 Nov., 2005) (NonPatent Document 4) discloses a method employing chemical vapordeposition to produce graphene. More specifically, camphor vapor isthermally decomposed at 700-850° C. at an Ni crystal face to obtaingraphene.

It is difficult, however, to use these methods to handle general,industrial production, and furthermore, the methods cannot provide alarge area of graphite film essential to device application.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Patent Laying-Open No. 2007-112662-   Patent Document 2: Japanese Patent Laying-Open No. 2007-63051-   Patent Document 3: Japanese Patent Laying-Open No. 2002-255528-   Patent Document 4: Japanese Patent Laying-Open No. 2003-238126-   Patent Document 5: Japanese Patent Laying-Open No. 2000-86219

Non Patent Document

-   Non Patent Document 1: K. S. Novoselov et. al., Science 306 (2004)    pp. 666-669.-   Non Patent Document 2: K. S. Novoselov et. al., Proc. Natl. Acad.    Sci. U.S.A. 102 (2005) pp. 10451-10453.-   Non Patent Document 3: C. Berger et. al., J. Phys. Chem. B108 (2004)    pp. 19912-19916.-   Non Patent Document 4: Yuanbo Zhang et. al., Nature 438, pp. 201-204    (10 Nov., 2005)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The above described conventional CNT production methods focus oncontrolling CNT as a single CNT in size or the like. When CNT'sindustrial application is considered, however, it would be necessary toassemble a plurality of such CNTs to produce elongate wire (or carbonwire). As the present inventors have studied, while a single CNT is ofsignificantly low resistance, such a plurality of CNTs (each having alength for example of several tens to several hundreds μm) collected(e.g., stranded) together to provide wire (or carbon wire) exhibit ascarbon wire an electrical resistance value higher by approximately 3digits than copper wire.

The present invention, has been made to overcome such disadvantage asdescribed above, and it contemplates a carbon wire employing CNT or asimilar carbon filament having a sufficiently low electrical resistancevalue, and a wire assembly employing the carbon wire.

Furthermore, the present invention also contemplates electricallyconductive film having a carbon nanotube network formed of a pluralityof low resistance carbon nanotubes linked together by graphite film, anelectrically conductive substrate and a transparent, electricallyconductive sheet employing the same, and a method for reproduciblyproducing the same.

Furthermore, the present invention also contemplates a method ofproducing graphite film that can facilitate synthesizing a large area ofgraphite film significantly reproducibly.

Means for Solving the Problems

According to the present invention, a carbon wire includes an assemblyportion and a graphite layer. The assembly portion is formed of aplurality of carbon filaments in contact with one another. The graphitelayer is provided at an outer circumference of the assembly portion.

Thus the carbon wire can have an outer circumferential graphite layerholding an assembly portion to ensure that the assembly portion has itscarbon filaments in contact with one another. This allows the assemblyportion to have the carbon filaments in contact with one another over anincreased area with increased pressure exerted to that area. This canprevent the assembly portion from having the carbon filaments in contactwith one another insufficiently and the carbon wire from accordinglyhaving an increased electrical resistance value. Furthermore, thegraphite layer can also act as an electrically conductive layer to allowthe carbon wire to have a further reduced electrical resistance value.

In the above carbon wire, the carbon filament may be a carbon nanotube.As the carbon nanotube exhibits satisfactory conductivity (or has a lowelectrical resistance value), the carbon wire can have a further reducedelectrical resistance value.

In the above carbon wire, the graphite layer may be a carbon nanotube.The graphite layer can also act as an electrically conductive layer, andcan thus more effectively reduce the carbon wire's electricalresistance.

According to the present invention, a wire assembly includes a pluralityof the above carbon wires. This allows the wire assembly to besufficiently low in resistance. Furthermore, the plurality of carbonwires allow the wire assembly to have a large area in cross section andaccordingly enable the wire assembly to pass a current of a large value.

According to the present invention, a method of producing a carbon wireincludes the steps of: preparing an assembly portion formed of aplurality of carbon filaments in contact with one another; and exposinga surface of the assembly portion to liquid gallium to provide agraphite layer on the surface of the assembly portion.

This allows an assembly portion at a portion of carbon filament that isexposed at a surface to be exposed to liquid gallium to provide agraphite layer through the liquid gallium's catalysis. When this iscompared for example with providing a graphite layer directly on asurface of the assembly portion through vapor deposition, the formerallows a process to be performed at a temperature lower than the latterto provide the graphite layer to obtain the present carbon wire.

In the above method of producing a carbon wire, the step of exposing maybe performed with compressive stress exerted to the assembly portion.This allows a graphite layer to be provided while compressive stress isexerted to the assembly portion, and the resultant carbon wire can havethe assembly portion configured of carbon filaments in contact with oneanother over an increased area with increased pressure exerted to thatarea. This more reliably ensures that the carbon wire has a reducedelectrical resistance value.

In the above method of producing a carbon wire, the step of exposing maybe performed with liquid gallium compressed to exert compressive stressto the assembly portion. The liquid gallium compressed (for example byincreasing the pressure of an ambient gas that is brought into contactwith the liquid gallium, or by enclosing Ga and CNTs in a capsule or asimilar container and then compressing them together with the capsule(or container) can facilitate exerting compressive stress to theassembly portion.

In the above method of producing a carbon wire, the step of exposing maybe performed with the liquid gallium in contact with ambient gasregulated in pressure to compress the liquid gallium. This canfacilitate compressing the liquid gallium. Furthermore, regulating theambient gas's pressure can facilitate regulating the value of thepressure applied to the liquid gallium.

In the above method of producing a carbon wire, the step of exposing isperformed with the liquid gallium having a temperature in a range of450° C.-750° C. This can more efficiently cause the liquid gallium'scatalytic reaction providing the graphite layer from an outercircumference of the assembly portion. Note that the liquid gallium'slower temperature limit is set at 450° C. because if the liquid galliumhas a temperature lower than that temperature, the liquid gallium'scatalytic reaction is insufficiently provided. Furthermore, the liquidgallium's upper temperature limit is set at 750° C. in order to preventthe assembly portion from having its constituent carbon filamentsdecomposed.

The above method of producing a carbon wire may include, before the stepof exposing, the step of providing an amorphous carbon layer as asurface layer of the assembly portion. Previously providing theamorphous carbon layer that is to serve as the graphite layer allows thegraphite layer to be provided while in the assembly portion the carbonfilaments' structure can be maintained. This allows an increased degreeof freedom in designing the carbon wire in configuration.

The above method of producing a carbon wire may further include, afterthe step of exposing, the step of removing gallium adhering to a surfaceof the carbon wire. The step of exposing may results in the carbon wirehaving a surface with the liquid gallium solidified and thus adheringthereto. The step of removing can remove such solidified gallium fromthe surface of the carbon wire.

According to the present invention, a method of producing a wireassembly includes the steps of: producing a plurality of carbon wires inthe above method of producing carbon wire; and stranding the pluralityof carbon wires together to form a wire assembly. Low-resistance carbonwires according to the present invention can thus be used to produce awire assembly.

Furthermore, the present invention provides electrically conductive filmhaving a carbon nanotube network formed of a plurality of carbonnanotubes linked together by graphite film.

The present invention provides a method of producing the electricallyconductive film, including the step of exposing a carbon nanotubenetwork to Ga vapor to provide the graphite film. Bulk Ga and carbon asseen in phase diagram are of a non solid solution type. However, thepresent inventors have found that in a microscale, Ga and carbon attheir surfaces have a bond caused and Ga vapor per se has a catalysisfor graphitization reaction.

The present invention provides a method of producing the electricallyconductive film, including the steps of: providing amorphous carbon filmon a carbon nanotube network; and exposing the carbon nanotube networkand the amorphous carbon film obtained in the step of providing, to Gavapor to provide the graphite film. The present inventors have foundthat Ga not only in the form of an aggregation of atoms as liquid butalso in the form of vapor having atoms liberated converts to a graphitestructure at a surface of amorphous carbon, i.e., that it causes agraphitization reaction of the surface of amorphous carbon. In otherwords, the present invention includes the step of causing Ga vapor toact on amorphous carbon or a similar carbon source to graphitize itssurface. Note that in the present invention the graphite film includesboth a graphene film in the form of a single layer, and a graphite filmformed of graphene films stacked in a plurality of layers.

According to the present invention, the method of producing theelectrically conductive film preferably includes, before the step ofexposing, the step of mechanically pressure-welding those portions of aplurality of carbon nanotubes forming the carbon nanotube network whichare in contact with one another.

The present invention provides an electrically conductive substrateformed with a substrate and electrically conductive film provided on thesubstrate and having a carbon nanotube network formed of a plurality ofcarbon nanotubes linked together by graphite film.

The present invention provides a method of producing the electricallyconductive substrate, including the steps of: forming a carbon nanotubenetwork on a substrate; and exposing the carbon nanotube network to Gavapor to provide the graphite film.

The present invention provides a method of producing the electricallyconductive substrate, including the steps of: forming a carbon nanotubenetwork on a substrate; providing amorphous carbon film on the carbonnanotube network; and exposing the carbon nanotube network and theamorphous carbon film that is obtained in the step of providing, to Gavapor to provide the graphite film.

According to the present invention, the method of producing theelectrically conductive substrate preferably includes, before the stepof exposing, the step of mechanically pressure-welding those portions ofa plurality of carbon nanotubes forming the carbon nanotube networkwhich are in contact with one another.

The present invention provides a transparent, electrically conductivesheet formed with a sheet of resin and electrically conductive filmprovided on the sheet of resin and having a carbon nanotube networkformed of a plurality of carbon nanotubes linked together by graphitefilm.

According to the present invention, preferably in the transparent,electrically conductive sheet, a surface of the sheet of resin that hasthe electrically conductive film is formed of one of thermosetting resinand ultraviolet (UV) curable resin.

The present invention provides a method of producing the transparent,electrically conductive sheet, including the steps of: forming a carbonnanotube network on a substrate; exposing the carbon nanotube network toGa vapor to provide the graphite film; and transferring to a sheet ofresin an electrically conductive film having the carbon nanotube networkformed of a plurality of carbon nanotubes linked together by graphitefilm in the step of exposing.

The present invention provides a method of producing the transparent,electrically conductive sheet, including the steps of: forming a carbonnanotube network on a substrate; providing amorphous carbon film on thecarbon nanotube network; exposing the carbon nanotube network and theamorphous carbon film that is obtained in the step of providing, to Gavapor to provide the graphite film; and transferring to a sheet of resinan electrically conductive film having the carbon nanotube networkformed of a plurality of carbon nanotubes linked together by graphitefilm in the step of exposing.

According to the present invention, the method of producing atransparent, electrically conductive sheet preferably includes, beforethe step of exposing, the step of mechanically pressure-welding thoseportions of the plurality of carbon nanotubes forming the carbonnanotube network which are in contact with one another.

According to the present invention, preferably, the method of producinga transparent, electrically conductive sheet includes the step oftransferring to transfer the electrically conductive film to the surfaceof the sheet of resin that is formed of one of thermosetting resin andultraviolet curable resin, and the method further includes the step ofsetting/curing one of the thermosetting resin and the ultravioletcurable resin.

Furthermore, the present invention provides a method of producinggraphite film by exposing a surface of a carbon source to Ga vapor toprovide graphite film on the surface of the carbon source.

Preferably, the Ga vapor has a temperature equal to or higher than 600°C. Ga vapor having a temperature of 600° C. or higher allowsgraphitization reaction to proceed satisfactorily.

Preferably, the Ga vapor has a uniform vapor pressure at the surface ofthe carbon source. This allows a graphite film to be provided with ahomogenous property.

Preferably, the Ga vapor is plasmatized.

Furthermore, preferably, the carbon source is located on a substrate andthe Ga vapor plasmatized is brought into contact with the substratehaving a temperature equal to or higher than 400° C.

Ga vapor plasmatized allows graphite film to be provided while thesubstrate having a source material of amorphous carbon applied thereonis maintained at as low a temperature as approximately 400° C.Semiconductor device processes require significantly strict temperaturerestrictions in order to maintain impurity profiles of channels,source/drain layers and the like. For example, approximately 500° C. orhigher temperatures cannot be set for processing. Plasmatized galliumallows catalysis to be exhibited at a temperature equal to or lower than400° C.

Preferably, the carbon source is amorphous carbon.

Preferably, the amorphous carbon is amorphous carbon film provided on amonocrystalline substrate formed of one type selected from the groupconsisting of SiC, Ni, Fe, Mo, and Pt.

For example, when a graphite film is provided on a silicon oxide film,the graphite film is not necessarily provided as monocrystalline filmand instead as polycrystalline film having a domain structure in a broadsense. An underlying substrate that is a SiC, Ni, Fe, Mo, Pt or similarcrystalline substrate allows graphite film to be provided asmonocrystalline film.

Preferably, the carbon source is a hydrocarbon material. In the presentinvention the carbon source other than amorphous carbon may be used,such as phenanthrene, pyrene, camphor or similar hydrocarbon materials.

According to the present invention, in the method of producing graphitefilm, the carbon source can be a three dimensional amorphous carbonstructure having a surface exposed to Ga vapor to provide graphite filmhaving a three dimensional surface structure.

For example, Ga vapor used as a catalyst can graphitize not onlyamorphous carbon in the form of a plane but also a surface of a pillaror a similar, three dimensional, any spatial geometry of amorphouscarbon.

Furthermore, the present invention relates to a method of producinggraphite film by mixing Ga vapor and a source material gas of a carbonsource together and supplying a mixture thereof to provide graphite filmon a substrate. This allows the substrate to have relatively thickgraphite film thereon.

Preferably, the Ga vapor has a temperature equal to or higher than 600°C. or higher.

Preferably, the Ga vapor is plasmatized.

Preferably, the Ga vapor plasmatized is brought into contact with thesubstrate having a temperature equal to or higher than 400° C.

Effects of the Invention

The present invention can achieve a low resistance carbon wire and a lowresistance wire assembly.

Furthermore, the present invention can achieve a low resistanceelectrically conductive film having a carbon nanotube network, and anelectrically conductive substrate and a transparent, electricallyconductive sheet utilizing the same.

The present invention also has a side effect including large lighttransmission. If fine particles or the like are used to provide asurface of a substrate with electrical conductivity, the particles mustbe closely packed to cover the surface of the substrate entirely. Incontrast, carbon nanotubes can eliminate the necessity of covering thesurface of the substrate entirely. The carbon nanotubes that do notentirely cover the surface of the substrate allow the substrate to havethe surface with many gaps, which can facilitate transmitting light.

Furthermore, the present method of producing graphite film is applicableto producing a transparent, electrically conductive sheet used for avariety of electronic devices, large-size displays and the like. Thepresent invention, for device applications, can facilitate efficientmass production of monocrystalline graphite film. Furthermore, thepresent invention, for transparent, electrically conductive sheet, canprovide means for obtaining a large area and number of layers ofgraphite film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section showing an embodiment of a carbonwire in the present invention.

FIG. 2 is a schematic cross section taken along a line II-II shown inFIG. 1.

FIG. 3 is a schematic cross section showing an embodiment of a wireassembly in the present invention.

FIG. 4 is a flowchart for illustrating a method of producing the FIG. 3wire assembly.

FIG. 5 is a flowchart for illustrating another method of producing awire assembly according to the present invention.

FIG. 6 is a schematic diagram for illustrating a coating step shown inFIG. 5.

FIG. 7 is a schematic diagram for illustrating a Ga catalyst reactionstep shown in FIG. 5.

FIG. 8 schematically shows a process for producing electricallyconductive film, an electrically conductive substrate, and atransparent, electrically conductive sheet according to the presentinvention.

FIG. 9 is a schematic cross section of one example of a graphite filmproduction apparatus used in the present invention.

FIG. 10 is a schematic cross section of one example of a graphite filmproduction apparatus used in the present invention.

FIG. 11 is a schematic cross section of one example of a graphite filmproduction apparatus showing a configuration of a subordinate Gareaction chamber.

FIG. 12 is a schematic cross section of one example of a graphite filmproduction apparatus employing Ga plasma.

FIG. 13 is a schematic cross section of one example of a graphite filmproduction apparatus with a carbon material supply system and a Gasupply system separated for forming a large area of transparent,electrically conductive sheet.

MODES FOR CARRYING OUT THE INVENTION

Hereinafter reference will be made to the drawings to describe thepresent invention in an embodiment. In the figures, identical orcorresponding components are identically denoted and will not bedescribed repeatedly in detail.

<Carbon Wire, Wire Assembly and their Production Methods>

FIG. 1 is a schematic cross section showing an embodiment of a carbonwire in the present invention. FIG. 2 is a schematic cross section takenalong a line II-II shown in FIG. 1. With reference to FIG. 1 and FIG. 2,the present invention provides a carbon wire 1, as will be describedhereinafter. Note that FIG. 1 shows carbon wire 1 in cross section asseen in a direction perpendicular to its longitudinal direction and FIG.2 shows carbon wire 1 in cross section as seen in a direction along itslongitudinal direction.

As shown in FIG. 1 and FIG. 2, carbon wire 1 includes an assemblyportion 3 and a graphite layer 4. Assembly portion 3 is configured of aplurality of carbon filaments implemented as carbon nanotubes 2 incontact with one another. Graphite layer 4 surrounds assembly portion 3.While FIG. 1 and FIG. 2 show carbon wire 1 configured, as seen in crosssection, of two carbon nanotubes 2, carbon wire 1 may have assemblyportion 3 configured, as seen in cross section, of two or more, e.g.,three or four carbon nanotubes (CNTs) 2. Furthermore, as shown in FIG. 1and FIG. 2, assembly portion 3 is configured of carbon nanotubes 2 incontact with one another. Furthermore, as shown in FIG. 2, carbon wire 1as seen in its longitudinal direction also has carbon nanotubes 2successively in contact with one another to allow assembly portion 3 tohave carbon nanotubes 2 forming an electrically conducting pathextending in the longitudinal direction of carbon wire 1 and capable ofpassing an electric current therethrough.

Carbon wire 1 can thus have an outer circumference formed of graphitelayer 4 holding assembly portion 3 to ensure that assembly portion 3 hascarbon nanotubes 2 in contact with one another. This allows assemblyportion 3 to have carbon nanotubes 2 in contact with one another over anincreased area with increased pressure exerted to that area. This canprevent assembly portion 3 from having carbon nanotubes 2 in contactwith one another insufficiently and hence carbon wire 1 from having anincreased electrical resistance value. Furthermore, graphite layer 4 canalso act as an electrically conductive layer allowing carbon wire 1 tohave a further reduced electrical resistance value.

Furthermore, carbon wire 1 including assembly portion 3 configured ofcarbon filaments formed of satisfactorily electrically conductive carbonnanotubes 2 ensures that carbon wire 1 has a reduced electricalresistance value.

Preferably, carbon wire 1 has graphite layer 4 formed of a carbonnanotube. In that case, graphite layer 4 can also act as an electricallyconductive layer, and carbon wire 1 can further be reduced in electricalresistance.

Furthermore, preferably, graphite layer 4 causes carbon nanotubes 2 thatconfigure assembly portion 3 to press one another. This allows assemblyportion 3 to have carbon nanotubes 2 in contact with one another over anincreased area with increased pressure exerted to that area, and alsoallows graphite layer 4 to contact carbon nanotubes 2 of assemblyportion 3 over an increased area with increased pressure exerted to thatarea. As a result, carbon wire 1 of low resistance can be implemented.

FIG. 3 is a schematic cross section showing an embodiment of a wireassembly in the present invention. With reference to FIG. 3, the presentinvention provides a wire assembly 5, as will be described hereinafter.Note that FIG. 3 shows wire assembly 5 in cross section as seen in adirection perpendicular to its longitudinal direction.

With reference to FIG. 3, wire assembly 5 includes a plurality of carbonwires 1 as described above (in FIG. 3, it includes seven carbon wires1). Thus, carbon wire 1 of low resistance according to the presentinvention can be used to implement wire assembly 5 of sufficiently lowresistance. Furthermore, using a plurality of carbon wires 1 allows wireassembly 5 to have a large area in cross section and hence pass acurrent having a large value. Furthermore, wire assembly 5 may have aplurality of carbon wires 1 twined together, or simply bundled and boundby a clamping member surrounding the plurality of carbon wires 1. Theclamping member may for example be an annular clamp formed for exampleof insulator (e.g., resin).

Note that wire assembly 5 may be configured of carbon wires 1 differentin number than as shown in FIG. 3 (for example, the wire assembly may beconfigured of two or any larger number of carbon wires). Furthermore,while FIG. 3 shows wire assembly 5 configured of carbon wires 1 allidentically structured, carbon wire 1 may be different in configurationfor some portion in cross section of wire assembly 5. For example, wireassembly 5 as seen in cross section may have a center portion withcarbon wire 1 configured of carbon nanotubes 2 (see FIG. 1) bundled inan increased number (as seen in a cross section in a directionperpendicular to that in which carbon wire 1 extends) (e.g., 10 or morecarbon nanotubes 2), while wire assembly 5 as seen in cross section mayhave an outer circumference with carbon wire 1 configured of carbonnanotubes 2 bundled in a number smaller than that of carbon nanotubes 2bundled that are located in carbon wire 1 at the center portion (e.g.,less than ten, more specifically, five or less carbon nanotubes 2 may bebundled together).

Furthermore, wire assembly 5 may be exposed to liquid gallium (a Gacatalyst), as done in the step of providing graphite layer 4 for carbonwire 1, as will be described hereinafter, to provide a graphite layersurrounding wire assembly 5. Furthermore, a plurality of wire assemblies5 each externally circumferentially surrounded by the graphite layer areprepared and bundled together to prepare a wire having a larger area incross section. Furthermore, the wire is also exposed to liquid galliumto have a graphite layer surrounding the wire. Furthermore, a pluralityof such wires each externally circumferentially surrounded by thegraphite layer are bundled together to configure a wire having a largerarea in cross section. Thus bundling wires together to provide a wireassembly, exposing the wire assembly to liquid gallium to provide agraphite layer on a surface of the wire assembly, and further bundling aplurality of such wire assemblies each having the graphite layerprovided thereon are repeated to produce a wire assembly further reducedin resistance and increased in diameter.

FIG. 4 is a flowchart for illustrating a method of producing the FIG. 3wire assembly. With reference to FIG. 4, the FIG. 3 wire assembly isproduced in the method, as will be described hereinafter.

As shown in FIG. 4, wire assembly 5 is produced in a method in which aCNT production step (S10) is first performed. In this CNT (carbonnanotube) production step (S10) a short (e.g., several μm long) carbonnanotube is produced in a conventionally well known method.

For example, a substrate used to produce a CNT is provided on a surfacethereof with an underlying film, and on the underlying film a pluralityof nanoparticles acting as a catalyst for forming a carbon nanotube areformed such that they are dispersed. The underlying film is configuredof material preferably for example of alumina, silica, sodium aluminate,alum, aluminum phosphate or a similar aluminum compound, calcium oxide,calcium carbonate, calcium sulfate or a similar calcium compound,magnesium oxide, magnesium hydroxide, magnesium sulfate or a similarmagnesium compound, or calcium phosphate, magnesium phosphate or asimilar apatite material. The nanoparticles can be configured ofactivated metal, such as vanadium (V), chromium (Cr), manganese (Mn),iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn) or the like.

Furthermore, the nanoparticles have a size for example equal to orsmaller than 100 nm, preferably 0.5 nm-10 nm, more preferably 1.0 nm-5nm. Furthermore, the underlying film can have a thickness for example of2.0 nm-100 nm.

A gas of a source material for forming a carbon nanotube is supplied toa surface of the substrate having the nanoparticles formed thereon, andthe substrate is heated in that condition. As a result, a carbonnanotube is grown on the surfaces of the nanoparticles disposed on thesurface of the substrate. The carbon nanotube thus grown is used to forman assembly portion configured of a plurality of carbon nanotubesassembled together, as will be described hereinafter.

Subsequently, as shown in FIG. 4, a CNT assembly formation step (S20) isperformed. In this step (S20), a conventionally well known method isused to strand a plurality of carbon nanotubes that are produced in thestep (S10) together to form an assembly portion formed of the carbonnanotubes. In this step (S20), a conventionally well known method can beused to form the assembly portion of the carbon nanotubes. For example,a required number of nano-size catalysts can be adjacently placed togrow carbon nanotubes (CNTs) to bond a required number of CNTs together,or furthermore, a plurality of CNTs may have their ends chucked androtated, and thus formed into a stranded wire.

Subsequently, a Ga catalyst reaction step (S30) is performed. In thisstep (S30), the assembly portion formed of the carbon nanotubes in thestep (S20) has a surface exposed to liquid gallium (Ga). As a result,the assembly portion formed of the carbon nanotubes has a surface layerconverted by the liquid gallium's catalytic reaction to a graphite layersurrounding the assembly portion. As a result, as shown in FIG. 1 andFIG. 2, carbon wire 1 having assembly portion 3 surrounded by graphitelayer 4 can be obtained. In other words, the steps (S10) to (S30)correspond to a method of producing carbon wire 1.

In doing so, the liquid gallium has a temperature of 450° C.-750° C.,more preferably 550° C.-700° C. This can more efficiently cause theliquid gallium's catalytic reaction forming the graphite layer at anouter circumference of assembly portion 3.

Furthermore, the step (S30) of exposing a surface of the assemblyportion to liquid gallium to provide a graphite layer is preferablyperformed while compressive stress is exerted to assembly portion 3.Providing graphite layer 4 while assembly portion 3 experiencescompressive stress allows carbon wire 1 to be formed with assemblyportion 3 configured of carbon nanotubes 2 in contact with one anotherover an increased area with increased pressure exerted to that area.This further ensures that carbon wire 1 and wire assembly 5 achieve areduced electrical resistance value.

Furthermore in the step (S30) preferably the liquid gallium iscompressed to exert compressive stress to assembly portion 3. Morespecifically, an ambient gas that contacts the liquid gallium may beregulated in pressure to compress the liquid gallium. For example, theliquid gallium may be held in a bath held in a holding container (achamber) and an ambient gas (that contacts the liquid gallium) in thatchamber may be regulated in pressure. The liquid gallium thus compressedcan facilitate exerting compressive stress to assembly portion 3.Furthermore, the ambient gas regulated in pressure can facilitateregulating a value in pressure applied to the liquid gallium. Note thatthe ambient gas can for example be argon gas, nitrogen gas or an inertgas less reactable with carbon nanotube and liquid gallium. Furthermore,the ambient gas's pressure can be set for example at gallium (Ga) vaporpressure to 10 Mpa, more preferably 1×10⁻⁵ torr to 1 Mpa.

Subsequently, an adhering-Ga removal step (S40) is performed. Morespecifically, after the graphite layer is provided, i.e., after the step(S30) is performed, carbon wire 1 has removed the gallium adhering onits surface, i.e., the adhering-Ga removal step (S40) is performed toremove gallium adhering to a surface of carbon wire 1 (i.e., a surfaceof graphite layer 4) provided. The gallium can be removed in any method.For example, a solution (e.g., diluted hydrochloric acid or dilutednitric acid) that can dissolve gallium may be sprayed to carbon wire 1,or a bath of the solution can be used to immerse carbon wire 1 therein.This can remove from a surface of carbon wire 1 the gallium solidifiedand thus adhering to the surface of carbon wire 1 in the step (S30).This can reduce in a post-step, or a processing step (S50), apossibility that otherwise solidified gallium causes a defect in formingwire assembly 5.

The steps (S10) to (S40) are performed a plurality of times or the step(S20) is performed to form a plurality of assembly portions formed ofcarbon nanotubes and the plurality of assembly portions concurrently andin parallel undergo the step (S30) and the step (S40) to obtain aplurality of carbon wires. Thus the steps (S10) to (S40) indicating amethod of producing a carbon wire are used to perform a process forproducing a plurality of carbon wires.

Subsequently, the processing step (S50) is performed to strand aplurality of carbon wires 1 that are obtained through the step (S10) tothe step (S40) together to form wire assembly 5 (see FIG. 3). In thisstep (S50), any conventionally well known method can be employed tostrand the plurality of carbon wires 1 together. For example, a requirednumber of nano-size catalysts can be adjacently placed to grow CNTs tobond a required number of CNTs together, or furthermore, a plurality ofCNTs may have their ends chucked and rotated, and thus formed into astranded wire. Wire assembly 5 formed of carbon wires 1, as shown inFIG. 3, and low in resistance, can thus be obtained.

The method of producing carbon wire 1 or wire assembly 5, as describedabove, allows a portion of a carbon nanotube(s) of assembly portion 3that is exposed at a surface to be exposed to liquid gallium to obtaingraphite layer 4 through the liquid gallium's catalysis (see FIG. 1 andFIG. 2), as has been described in the step (S30). When this is comparedfor example with providing graphite layer 4 directly on a surface ofassembly portion 3 through vapor deposition, the former allows a processto be performed at a temperature lower than the latter to providegraphite layer 4 to provide the present carbon wire.

FIG. 5 is a flowchart for illustrating another method of producing awire assembly according to the present invention. FIG. 6 is a schematicdiagram for illustrating a coating step shown in FIG. 5. FIG. 7 is aschematic diagram for illustrating a Ga catalyst reaction step shown inFIG. 5. With reference to FIG. 5 to FIG. 7, the present inventionprovides the wire assembly produced in the other method, as will bedescribed hereinafter.

The FIG. 5 wire assembly production method includes steps basicallysimilar to those of the FIG. 4 wire assembly production method, exceptthat the former has the Ga catalyst reaction step (S30) preceded by thestep of providing an amorphous carbon layer as a surface layer of anassembly portion, i.e., a coating step (S60).

More specifically, in the FIG. 5 wire assembly production method, thestep (S10) and the step (S20) are initially performed, as done in theFIG. 4 wire assembly production method, and thereafter, as shown in FIG.6, on a surface of assembly portion 3 obtained, an amorphous carbonlayer 11, which is to serve as graphite layer 4 (see FIG. 7), isprovided. Amorphous carbon layer 11 can be provided in any conventionalwell known method. For example, phenanthrene (C₁₄H₁₀), pyrene, methaneacetylene or the like may be thermally decomposed to provide amorphouscarbon layer 11, or an electron beam or an ion beam may be used todecompose a hydrocarbon based gas. As a result, the FIG. 6 structure isobtained.

Subsequently, as shown in FIG. 5, the Ga catalyst reaction step (S30) isperformed. This step (S30) can be performed in a method basicallysimilar to the step (S30) performed in the FIG. 4 production method. Itshould be noted, however, that in the FIG. 5 step (S30), amorphouscarbon layer 11 has a surface layer converted to graphite layer 4through the liquid gallium's catalytic reaction. As a result, carbonwire 1 having a structure shown in FIG. 7 can be obtained.

The FIG. 5 production method can thus provide graphite layer 4 fromamorphous carbon layer 11 while maintaining a structure of carbonnanotubes 2 in assembly portion 3. The method thus allows an increaseddegree of freedom in designing carbon wire 1 in configuration.

Subsequently, as done in the FIG. 4 production method, the step (S40)and the step (S50) can be performed to obtain a wire assembly similar instructure to the FIG. 3 wire assembly 5. Note that the FIG. 5 productionmethod produces a wire assembly configured of carbon wire 1 havingamorphous carbon layer 11 posed between carbon nanotube 2 configuringassembly portion 3 and graphite layer 4, as can be seen in FIG. 7.

<Electrically Conductive Film, Electrically Conductive Substrate, andTransparent, Electrically Conductive Sheet>

FIG. 8 schematically shows a process for producing electricallyconductive film, an electrically conductive substrate, and atransparent, electrically conductive sheet according to the presentinvention. Initially, as shown in FIG. 8 (a), a substrate 17 is exposedto a slurry having carbon nanotubes (hereinafter referred to as CNT(s))2 dispersed therein to form a carbon nanotube network (hereinafterreferred to as a CNT network) formed of a plurality of CNTs. The carbonnanotube network has a surface exposed to Ga vapor to allow the CNTnetwork to have its constituent CNTs linked together by graphite film toobtain electrically conductive film 18 and an electrically conductivesubstrate (FIG. 8 (b)). Furthermore, a resin sheet is brought intocontact, at a surface thereof having thermosetting resin or ultraviolet(UV) curable resin thereon, with the surface of electrically conductivefilm 18 that has the CNT network formed thereon, and the resin sheet isthen thermally set or UV-cured to transfer the CNT network to the resinsheet to obtain the present transparent, electrically conductive sheet(FIG. 8 (c)).

(Carbon Nanotube)

Carbon nanotube 2, which is a tube having a lattice structure ofcarbocyclic six-membered rings, may be a tube structured of a singlesheet, i.e., a single-walled carbon nanotube (hereinafter also referredto as “SWNT”), or may be a tube structured of multiple layers of tubeshaving the lattice structure of carbocyclic six-membered rings, i.e., amultiwalled-carbon nanotube (hereinafter also referred to as “MWNT”). Ingeneral, an SWNT is more flexible. An MWNT is less flexible than theSWNT, and MWNTs having more multiple layers have a tendency to be morerigid. It is desirable that an SWNT or an MWNT be used depending on thepurpose, as occasion demands, with their properties considered.

In what length the carbon nanotube is applicable is not limited to anyparticular value. In general, however, a carbon nanotube in a range of10 nm to 1000 μm, preferably 100 nm to 100 μm, is used. The carbonnanotube is not limited in diameter (or thickness) to any particularvalue. In general, however, a carbon nanotube in a range of 1 nm to 50nm is used, and for an application requiring more transparency, a carbonnanotube in a range of 3 nm to 10 nm is preferably used.

Note that when carbon nanotubes are applied to substrate 17, it ispreferable to previously prepare a slurry having the CNTs dispersedtherein. The slurry is prepared as follows: CNTs prepared in an arcprocess are introduced in acetone and bundled CNTs are ultrasonicallydebundled and dispersed uniformly in the acetone. Subsequently, beforetime elapses, the slurry is sprayed to substrate 17 and dried to form aCNT network on the substrate. The acetone may be replaced with alkylbenzene sulfonate or a similar surfactant, a sulfosuccinate diester or asimilar solvent having a structure of a hydrophobic moiety-a hydrophilicmoiety-a hydrophobic moiety to be used to similarly disperse CNTstherein. In that case, a dispersant or the like will enter between thoseportions of CNTs at which the CNTs contact one another. Accordingly,preferably, after the slurry is dried on the substrate, water or acetoneis used to wash away the dispersant or other matters adhering to thesubstrate.

(Carbon Nanotube Network)

A carbon nanotube network is formed of a plurality of CNTs randomlyintertwined with one another on substrate 17 and thus formed in anetwork. A conventional CNT network is large in electrical resistance,as it has its CNTs electrically connected only by the physical contactmade at those portions of the CNTs at which the CNTs contact oneanother. The present invention allows a CNT network to be processed withGa vapor to provide CNTs with a graphite film on their surfaces to linkthe CNTs together. This can reduce the CNT network's electricalresistance and thus provide electrically conductive film having a lowresistance value.

CNTs may be brought into contact with a substrate to form a CNT networkin any method. They may be applied in any of generally used methods.Applicable methods include spin-coating, dip-coating, curtain-coating,roll-coating, applying with a brush, spray-coating and the like, forexample. In particular, spin-coating is preferable, as it can easilyprovide a CNT network in a homogeneous thin film.

(Substrate)

Substrate 17 may be any substrate that is normally used for productionof electrically conductive film. For example, a substrate formed ofglass, mica, quartz or a similar transparent material allows anelectrically conductive substrate as a whole to have significantlyincreased transparency. While it is known to employ carbon vapordeposition, metal vapor deposition or the like to provide a surface of asubstrate with electrical conductance, employing a carbon nanotubenetwork to provide a surface of substrate 17 with electricalconductance, as described in the present invention, can eliminate thenecessity of completely covering the surface with carbon nanotube. Thenetwork has gaps, and thus allows an electrically conductive substrateto have significantly high optical transmittance for a predeterminedsurface conductivity.

(Electrically Conductive Film)

Electrically conductive film 18 according to the present invention iselectrically conductive film having a carbon nanotube network formed ofa plurality of carbon nanotubes linked together by graphite film. Theelectrically conductive film has the CNTs electrically connected via thegraphite film, and thus has a low resistance value characteristically.

(Electrically Conductive Substrate)

The electrically conductive substrate according to the present inventionis an electrically conductive substrate formed of substrate 17 andelectrically conductive film 18 provided on substrate 17 and having acarbon nanotube network formed of a plurality of carbon nanotubes linkedtogether by graphite film. The electrically conductive substrate hasCNTs thereon electrically connected via the graphite film, and thus hasa low resistance value characteristically.

(Resin Sheet)

Resin sheet 4 may be of any highly transparent resin that is normallyused as a substrate. Preferably, polymeric (PET) film having epoxy resinor similar thermosetting resin, or acrylic syrup or similar UV curableresin or similar curable resin applied thereon is used, as it allows theCNT network formed on the electrically conductive film to be transferredefficiently.

The method of producing electrically conductive film, an electricallyconductive substrate and a transparent, electrically conductive sheet inaccordance with the present invention will more specifically bedescribed hereinafter for the step of providing graphite film on a CNTnetwork.

<Electrically Conductive Film, Electrically Conductive Substrate,Transparent, Electrically Conductive Sheet, and Method of Producing theSame>

FIG. 9 is a schematic cross section of one example of a graphite filmproduction apparatus used in the present invention.

(Graphite Film Production Apparatus)

The present invention employs a graphite film production apparatusconfigured of a quartz reaction tube 6 and an alumina container 20provided in quartz reaction tube 6 and having liquid Ga 9 introducedtherein. Substrate 17 with a plurality of carbon nanotubes 2 formedthereon in a CNT network is to be processed, placed in a vicinity ofalumina container 20. External to quartz reaction tube 6, a heater 7 isprovided for the reaction tube to regulate the internal temperature ofquartz reaction tube 6.

(Preparing Substrate to be Processed)

Substrate 17 may be a conventionally well known substrate that isnormally used for production of electrically conductive film. However, asubstrate formed of glass, mica, quartz or a similar transparentmaterial allows an electrically conductive film as a whole to havesignificantly increased transparency.

The CNT network formed of a plurality of carbon nanotubes 2 may beformed in any conventional well known method. For example, the methodincludes spin-coating, dip-coating, curtain-coating, roll-coating;applying with a brush, spray-coating and the like, for example. Inparticular, spin-coating is preferable, as it can easily provide a CNTnetwork in a homogeneous thin film. Subsequently, preferably, thesubstrate is washed to prevent the CNT network from having a dispersantor similar impurity remaining therein.

To bring the CNTs into close contact with one another, a roller or thelike is preferably used to firmly compress the CNT network from above.

Furthermore, preferably, amorphous carbon film is provided on the CNTnetwork to ensure that graphite film is provided. The amorphous carbonfilm may be provided in any conventional well known method. For example,phenanthrene (C₁₄H₁₀), pyrene, methane acetylene or the like may bethermally decomposed to provide the amorphous carbon film, or anelectron beam or an ion beam may be used to decompose a hydrocarbonbased gas. The amorphous carbon film preferably has a thickness equal toor smaller than 10 nm, as such film can enhance transparency.

(Method of Producing Electrically Conductive Film)

Initially, a substrate to be processed, as aforementioned, is secured inquartz reaction tube 6 horizontally, and a turbo pump is used to vacuumthe background to 10⁻⁶ Torr or lower.

Heater 7 heats the reaction tube to evaporate liquid Ga 9 in quartzreaction tube 6 and heats Ga vapor 5 to 600° C. or higher to bring thevapor into contact with a surface of the CNT network formed of aplurality of carbon nanotubes 2. More preferably, Ga vapor 5 is heatedto 800° C. or higher to enhance the catalysis of Ga vapor 5.

The above heat treatment is conducted for 10 minutes to 1 hour, andsubsequently the reaction tube is slowly cooled to again attain roomtemperature.

The heat treatment in Ga vapor 5 provides graphite film on a surface ofthe CNT network formed of carbon nanotubes 2. Thus, on substrate 17,electrically conductive film having a carbon nanotube network formed ofa plurality of carbon nanotubes linked together by graphite film isprovided, and an electrically conductive substrate is thus obtained.

(Method of Producing Transparent, Electrically Conductive Sheet)

The electrically conductive film produced in the aforementioned processis used to produce a transparent, electrically conductive sheet in amethod, as will be described hereinafter.

A sheet of resin is brought into contact with that surface of theaforementioned electrically conductive substrate which has the CNTnetwork formed thereon to transfer the CNT network to the sheet ofresin. Preferably, thermosetting resin or UV curable resin is applied tothat surface of the sheet of resin which is brought into contact withthe CNT network. Subsequently, the sheet of resin is set/cured to securethe CNT network to the sheet of resin to produce a transparent,electrically conductive sheet.

<Method of Producing Graphite Film>

First Embodiment

FIG. 10 is a schematic cross section of one example of a graphite filmproduction apparatus used in the present invention.

(Graphite Film Production Apparatus)

The present invention employs a graphite film production apparatusconfigured of quartz reaction tube 6 and alumina container 20 providedin quartz reaction tube 6 and having liquid Ga 1 introduced therein.Substrate 17 with amorphous carbon film 21 provided thereon is to beprocessed, placed in a vicinity of alumina container 20. External toquartz reaction tube 6, heater 7 is provided for the reaction tube toregulate the internal temperature of quartz reaction tube 6.

Substrate 17 may be a conventionally well known substrate that is usedas a substrate for production of electrically conductive film.Preferably, an SiC, Ni, Fe, Mo, Pt or similar, monocrystalline substrateis used, as monocrystalline graphite film can be obtained.

Amorphous carbon film 21 may be provided in any conventional well knownmethod. For example, phenanthrene (C₁₄H₁₀), pyrene, methane acetylene orthe like may be thermally decomposed to provide amorphous carbon film 2,or an electron beam or an ion beam may be used to decompose ahydrocarbon based gas. Amorphous carbon film 21 preferably has athickness set to match that of graphene film or graphite film targeted.

(Method of Producing Graphite Film)

Initially, a substrate to be processed, as aforementioned, is secured inquartz reaction tube 6 horizontally, and a turbo pump is used to vacuumthe background to 10⁻⁶ Torr or lower.

Heater 7 heats the reaction tube to evaporate liquid Ga 9 in quartzreaction tube 6 and heats Ga vapor 5 to 600° C. or higher to bring thevapor into contact with a surface of amorphous carbon film 21.

The above heat treatment is conducted for 10 minutes to 1 hour, andsubsequently the reaction tube is slowly cooled to again attain roomtemperature.

The heat treatment in Ga vapor 5 provides graphite film on a surface ofamorphous carbon film 21.

Second Embodiment

(Graphite Film Production Apparatus)

FIG. 11 is a schematic cross section of one example of a graphite filmproduction apparatus used in the present invention when Ga vapor hasuniform vapor pressure at a surface of a carbon source. A secondembodiment employs a graphite film production apparatus having quartzreaction tube 6 and a subordinate Ga reaction chamber 22 provided inquartz reaction tube 6 and accommodating alumina container 20 havingliquid Ga 9 introduced therein, and substrate 17 having amorphous carbonfilm 21 thereon, i.e., a substrate to be processed. Subordinate Gareaction chamber 22 has a wall having a differential evacuation in theform of a small gap.

The first embodiment shows a graphite film production apparatus havingquartz reaction tube 6 internally filled with Ga vapor 5 generated fromliquid Ga 9. However, while a portion of quartz reaction tube 6 that isclose to heater 7 is maintained at a predetermined high temperature,portions of quartz reaction tube 6 that are remoter from heater 7 arelower in temperature, and some of them have room temperature. Thus, inquartz reaction tube 6, Ga vapor 5 varies in temperature at differentlocations and thus does not have uniform vapor pressure.

As shown in FIG. 11, quartz reaction tube 6 that has subordinate Gareaction chamber 22 therein allows Ga vapor 5 to be held in subordinateGa reaction chamber 22 and thus have a fixed vapor pressure.Furthermore, subordinate Ga reaction chamber 22 that accommodatestherein alumina container 20 having liquid Ga 9 introduced therein, andsubstrate 17 having amorphous carbon film 21 thereon, or a substrate tobe processed, and that is vacuumed through a small gap serving as adifferential evacuation port, can internally have a Ga vapor pressure ofa possible maximal value and also provide a uniform Ga vapor pressure ina vicinity of the substrate to be processed. The aforementionedproduction method can provide graphite film having a surface withoutinconsistency in color or roughness and thus having a significantlysmooth mirror surface.

(Method of Producing Graphite Film)

Initially, a substrate to be processed, as aforementioned, is secured inquartz reaction tube 6 horizontally, and a turbo pump is used to vacuumthe background to 10⁻⁶ Torr or lower.

Heater 7 heats the reaction tube to evaporate liquid Ga 9 in quartzreaction tube 6 and heats Ga vapor 5 to 600° C. or higher to bring thevapor into contact with a surface of amorphous carbon film 21. Morepreferably, Ga vapor 5 is heated to 800° C. or higher to enhance thecatalysis of Ga vapor 5.

The heat treatment in Ga vapor 5 provides graphite film on a surface ofamorphous carbon film 21.

Third Embodiment

(Graphite Film Production Apparatus)

FIG. 12 is a schematic cross section of one example of a graphite filmproduction apparatus employed in the present invention with Ga vaporplasmatized. A third embodiment employs a graphite film productionapparatus having quartz reaction tube 6 accommodating alumina container20 having liquid Ga 9 introduced therein, and a plasma producingelectrode 10, with a heater 12 provided at the alumina container for Ga.Substrate 17 with amorphous carbon film 21 thereon, or a substrate to beprocessed, is positioned in a vicinity of alumina container 20 betweenpaired plasma producing electrodes 10 and exposed to a Ga plasma 23.External to quartz reaction tube 6, heater 7 is provided for thereaction tube to regulate the internal temperature of quartz reactiontube 6.

Using Ga vapor to obtain graphite film is an effective technique toobtain a single or multiple layers of large-area graphite film and is apractical technique directed to electronics device applications. Toobtain a transparent, electrically conductive sheet or a similar,electrically conductive film having a large area and a low resistancevalue, however, a process using Ga vapor must be performed a pluralityof times to repeat a reaction until electrically conductive film aspredetermined is obtained.

As shown in FIG. 12, Ga vapor can be plasmatized and thus provided withenergy to serve as a catalyst to graphitize amorphous carbon. When thisis compared with using Ga vapor, the former can provide graphite filmlarger in thickness. Furthermore, using Ga plasma allows graphitizationto be observed on a substrate having as low a temperature asapproximately 400° C., and can thus induce graphitization at a lowertemperature. For use with a silicon device process, graphite film mustbe provided directly on a silicon device, and accordingly, it isessential to perform the process at low temperature. In this view,plasmatizing Ga vapor to allow graphite film to be provided in a processperformed at low temperature is significantly effective for integrationwith the silicon device process.

(Method of Producing Graphite Film)

Initially, a substrate to be processed, as aforementioned, is secured inquartz reaction tube 6 horizontally, and a turbo pump is used to vacuumthe background to 10⁻⁶ Torr or lower.

Heater 12 for Ga is operated to facilitate evaporating liquid Ga 9,while plasma producing electrodes 10 are used to plasmatize Ga vaporpresent at a location sandwiched between the electrodes, and heater 7for the reaction tube is also used to heat the substrate exposed to Gaplasma 11 to 400° C. or higher and Ga plasma 23 is brought into contactwith a surface of amorphous carbon film 21. To enhance the catalysis ofGa plasma 23, the substrate to be processed in contact with Ga plasma 23has a temperature more preferably of 800° C. or higher.

The heat treatment in Ga plasma 23 converts amorphous carbon film 21 atleast partially or entirely to graphite film.

Fourth Embodiment

(Graphite Film Production Apparatus)

FIG. 13 is a schematic cross section of one example of a graphite filmproduction apparatus employed in the present invention with a carbonsource of a hydrocarbon gas. A fourth embodiment employs a graphite filmproduction apparatus having quartz reaction tube 6 connected to a Gavapor supply unit 15 and a hydrocarbon gas supply unit 13. Ga vaporsupply unit 15 receives liquid Ga 9, which is heated by a heater for Gaand thus evaporated to supply Ga vapor 5 to the interior of quartzreaction tube 6. Hydrocarbon gas supply unit 13 receives hydrocarbonmaterial serving as carbon material, such as camphor, phenanthrene,pyrene or the like, to supply a carbon source as hydrocarbon gas to theinterior of quartz reaction tube 6. Quartz reaction tube 6 accommodatessubstrate 17 therein as a substrate to be processed.

Quartz reaction tube 6 receives the hydrocarbon gas, which reacts withGa vapor in a vicinity of substrate 17, therewhile the gas is decomposedand thus rapidly forms graphite film on substrate 17.

When Ga is used to provide graphite film on a substrate, the Ga candisadvantageously be introduced into the film. When the substrate has atemperature of 600° C. or higher, the Ga is hardly introduced into thefilm. When the substrate has a low temperature of 600° C. or lower, andthe Ga is accordingly introduced into the graphite film, annealing atapproximately 500° C. for a long period of time allows Ga to beseparated and thus removed from the film.

(Method of Producing Graphite Film)

Initially, a substrate to be processed, as aforementioned, is secured inquartz reaction tube 6 horizontally, and a turbo pump is used to vacuumthe background to 10⁻⁶ Torr or lower.

Heater 12 for Ga is operated to evaporate liquid Ga 9 to supply Ga vaporto the interior of quartz reaction tube 6, while a valve 16 locatedbetween hydrocarbon gas supply unit 13 and quartz reaction tube 6 isopened to supply hydrocarbon gas.

Heater 7 for the reaction tube is operated to heat quartz reaction tube6 to heat Ga vapor 5 therein to 400° C. or higher and bring Ga vapor 5into contact with a surface of substrate 3. To enhance the catalysis ofGa vapor 5, Ga vapor 5 has a temperature more preferably of 800° C. orhigher.

The heat treatment in Ga vapor 5 provides graphite film on substrate 17.

EXAMPLES Example 1 of the Present Invention

Preparing Sample

An arc process is employed to produce unpurified single-walled carbonnanotubes (CNTs), which are used to form an assembly portion provided asa 0.3 mm-diameter wire formed of stranded CNT filaments. The wire formedof stranded CNT filaments has a length of 10 mm.

The wire formed of stranded CNT filaments is immersed for one hour inliquid gallium (Ga) heated to 600° C. In doing so, an ambient gas of Aris used, set at a pressure of 1×10⁻⁵ Torr.

Subsequently, the wire formed of stranded CNT filaments is drawn out ofthe liquid Ga and has Ga that adheres to its surface removed therefromwith diluted hydrochloric acid. The wire formed of stranded CNTfilaments is thus provided with a graphite layer on a surface thereof,i.e., a carbon wire is obtained. The graphite layer has a thickness ofapproximately 5 μm.

Measurement

The carbon wire having the graphite layer is subjected to measurement ofelectrical resistance by a 4-terminal method.

Result

The carbon wire having the graphite layer has a value in electricalresistance decreased to approximately ⅕ of that of a sample of acomparative example 1 described later.

Example 2 of the Present Invention

Preparing Sample

An arc process is employed to produce unpurified single-walled carbonnanotubes (CNTs), which are used to form an assembly portion provided asa 5 μm-diameter wire formed of stranded CNT filaments. The wire formedof stranded CNT filaments has a length of 10 mm.

Then, phenanthrene (C₁₄H₁₀) is thermally decomposed to provide anamorphous carbon layer on a surface of the wire formed of stranded CNTfilaments.

Subsequently the wire formed of stranded CNT filaments is immersed forone hour in liquid gallium (Ga) heated to 600° C. In doing so, anambient gas of Ar is used, set at a pressure of 2 atmospheres.

Subsequently, the wire formed of stranded CNT filaments is drawn out ofthe liquid Ga and has Ga that adheres to its surface removed therefromwith diluted hydrochloric acid. The wire formed of stranded CNTfilaments is thus provided with a graphite layer on a surface thereof,i.e., a carbon wire is obtained. The graphite layer has a thickness ofapproximately 1 μm.

Measurement

The carbon wire having the graphite layer is subjected to measurement ofelectrical resistance by a 4-terminal method.

Result

The carbon wire having the graphite layer has a value in electricalresistance decreased to approximately 1/20 of that of the comparativeexample for example 1 of the present invention.

From this result, the carbon wire is considered to have an interiorhaving a plurality of carbon nanotubes substantially integratedtogether.

Example 3 of the Present Invention

Preparing Sample

10 nm-diameter, 300 μm long carbon nanotubes (CNTs) are prepared andoverlapped by 100 μm to prepare an assembly portion implemented as awire formed of bonded CNT filaments. The wire formed of bonded CNTfilaments has a length of 50 mm and a diameter of 2 μm.

Then, phenanthrene (C₁₄H₁₀) is thermally decomposed to provide anamorphous carbon layer on a surface of the wire formed of bonded CNTfilaments.

Subsequently the wire formed of bonded CNT filaments is immersed for onehour in liquid gallium (Ga) heated to 500° C. In doing so, an ambientgas of argon (Ar) is used, set at a pressure of 10 atmospheres in orderto allow the wire to have its internal carbon nanotubes brought intoclose contact with one another.

Subsequently, the wire formed of bonded CNT filaments is drawn out ofthe liquid Ga and has Ga that adheres to its surface removed therefromwith diluted hydrochloric acid. The wire formed of bonded CNT filamentsis thus provided with a graphite layer on a surface thereof, i.e., acarbon wire is obtained. The graphite layer has a thickness ofapproximately 0.2 μm. Furthermore, it has been found that the graphitelayer is formed in successive rings wrapping the internal CNTs and hasthus become carbon nanotube (CNT).

Measurement

The carbon wire having the graphite layer is subjected to measurement ofelectrical resistance by a 4-terminal method.

Result

The carbon wire having the graphite layer has a value in electricalresistance smaller than the comparative example by one digit. This isprobably because the carbon wire has an interior having carbon nanotubesin close contact with one another and thus integrated together.

Example 4 of the Present Invention

Preparing Sample

Catalytic CVD is employed to prepare a 30 nm-diameter, 500 μm longmultiwalled carbon nanotube (CNT), and such CNTs are overlapped by 200μm to form an assembly portion implemented as a wire formed of bondedCNT filaments. The wire formed of bonded CNT filaments has a length of10 mm and a diameter of 0.6 μm.

Subsequently the wire formed of bonded CNT filaments is immersed for onehour in liquid gallium (Ga) heated to 550° C. More specifically, theliquid gallium and the wire formed of bonded CNT filaments are enclosedin a capsule of stainless steel. The capsule is surrounded by an ambientgas of argon (Ar). The ambient gas is compressed to exert pressure tothe liquid gallium and the wire of bonded CNT filaments together withthe capsule. The pressure is set at 100 atmospheres to allow the wire ofbonded CNT filaments to have its internal carbon nanotubes brought intoclose contact with one another.

Subsequently, the wire formed of bonded CNT filaments is drawn out ofthe liquid Ga and has Ga that adheres to its surface removed therefromwith diluted hydrochloric acid. The wire formed of bonded CNT filamentsis thus provided with a graphite layer on a surface thereof, i.e., acarbon wire is obtained. The graphite layer has a thickness ofapproximately 80 nm. Furthermore, it has been found that the graphitelayer is formed in successive rings wrapping the internal CNTs and hasthus become carbon nanotube (CNT).

Furthermore, such carbon wires each having the graphite layer arebundled and stranded together to provide a stranded wire to produce a0.5 μm-diameter stranded wire. Then, as has been done in anabove-described step, the stranded wire is immersed in liquid Ga to bondtogether a plurality of carbon wires configuring the stranded wire(i.e., a graphite layer is formed on a surface of a bundle of aplurality of carbon wires to surround the bundle). Thus the steps of:bundling a plurality of carbon wires together; immersing the wire of theassembly of the bundled carbon wires in liquid Ga to bond the pluralityof carbon wires together; and preparing and bundling together aplurality of such wires each of the assembly of the bonded carbon wiresare repeated to produce a wire having a larger diameter (morespecifically, a 0.1 mm-diameter wire formed of bonded wires).

Measurement

The carbon wire having the graphite layer is subjected to measurement ofelectrical resistance by a 4-terminal method.

Result

The carbon wire having the graphite layer has a value in electricalresistance smaller than the comparative example by two digits or larger.This is probably because the carbon wire has an interior having carbonnanotubes in close contact with one another and thus integratedtogether.

Comparative Example 1

Preparing Sample

An arc process is employed to produce unpurified, single-walled carbonnanotubes (CNTs) which are in turn used to form an assembly portionimplemented as a 0.3 mm-diameter wire formed of stranded CNT filaments.The wire formed of stranded CNT filaments has a length of 10 mm.

Measurement

The wire formed of stranded CNT filaments is subjected to measurement ofelectrical resistance by a 4-terminal method.

Result

The comparative example's wire formed of stranded CNT filaments has avalue in electrical resistance of 7.8×10⁻³ Ω·cm. This value is largerthan that of copper by three digits or larger.

Comparative Example 2

Preparing Sample

An arc process is employed to produce unpurified, single-walled carbonnanotubes (CNTs) which are in turn used to form an assembly portionimplemented as a 0.3 mm-diameter wire formed of stranded CNT filaments.The wire formed of stranded CNT filaments has a length of 10 mm.

The wire formed of stranded CNT filaments is immersed for one hour inliquid gallium (Ga) heated to 800° C. In doing so, an ambient gas of Aris used, set at a pressure of 1×10⁻⁵ Torr.

Result

The wire of stranded CNT filaments immersed in the liquid Ga, asdescribed above, has been decomposed in the liquid Ga and thusdisappeared. Accordingly, it is preferable that the wire formed ofstranded CNT filaments be immersed in liquid gallium heated to atemperature lower than 800° C., more preferably 750° C. or lower.

Examples 5, 6. Of the Present Invention, and Comparative Examples 3, 4

A ceramic substrate is prepared and thereon a slurry having CNTsdispersed therein is sprayed and then dried to form a CNT network on thesubstrate.

For example 5 of the present invention and comparative example 3, anamorphous carbon film of approximately 5 nm on average is provided onthe CNT network by laser abrasion.

Subsequently, for examples 5, 6 of the present invention, the FIG. 9graphite film production apparatus is employed to produce graphite film.

A 1 m long, 25 mm-diameter quartz tube is prepared as quartz reactiontube 6. Quartz reaction tube 6 accommodates an approximately 1cm-diameter alumina container having liquid Ga 9 introduced therein, andin a vicinity thereof, substrate 17 bearing a plurality of carbonnanotubes 2 forming a CNT network is placed as a substrate to beprocessed.

Initially, a substrate to be processed, as aforementioned, is secured inquartz reaction tube 6 horizontally, and a turbo pump is used to vacuumthe background to 10⁻⁶ Torr or lower.

Heater 7 for the reaction tube is operated to heat Ga vapor 5 to 650° C.to perform a process for one hour and subsequently the reaction tube isslowly cooled again to room temperature. Note that comparative examples3 and 4 are sample substrates which do not undergo the process with Gavapor.

The heat treatment with Ga vapor provides graphite film on a surface ofthe CNT network. For each example, the treatment's temperature and theprocessed substrate's sheet resistance value are as shown in table 1.Note that the sheet resistance value is measured in a 4-terminal method.

Furthermore, for each example, on the electrically conductive substratehaving the CNT network having a surface with the graphite film thereon,a resin sheet having thermosetting resin on a surface that is broughtinto contact with the CNT network is deposited from above, and thenthermally set to transfer and secure the CNT network to thethermosetting resin to obtain a transparent, electrically conductivesheet, which has a sheet resistance value as shown in table 1.

TABLE 1 Examples of the Comparative Present Invention Examples 5 6 3 4Amorphous carbon film + − + − Ga vapor process + + − − Ga processtemperature (° C.) 650 650 − − Process time 1 hour 1 hour − −Transparent, electrically  10  20 200 500 conductive sheet's sheetresistance value (kΩ/square)

Examples 7, 8 of the Present Invention, and Comparative Examples 5, 6

A glass substrate is prepared and thereon a slurry having CNTs dispersedtherein is sprayed and then dried to form a CNT network on thesubstrate. The substrate is washed with water and acetone to prevent adispersant or a similar impurity from remaining on the CNT network.

Subsequently, for example 7 of the present invention and comparativeexample 5, an organic gas (phenanthrene (C₁₄H₁₀)) is decomposed toprovide amorphous carbon film on the CNT network.

Subsequently, for examples 7, 8 of the present invention, the FIG. 9graphite film production apparatus is employed to produce graphite film.

A 1 m long, 25 mm-diameter quartz tube is prepared as quartz reactiontube 6. Quartz reaction tube 6 accommodates an approximately 1cm-diameter alumina container having liquid Ga 9 introduced therein, andin a vicinity thereof, substrate 17 bearing a plurality of carbonnanotubes 2 forming a CNT network is placed as a substrate to beprocessed.

Initially, a substrate to be processed, as aforementioned, is secured inquartz reaction tube 6 horizontally, and a turbo pump is used to vacuumthe background to 10⁻⁶ Torr or lower.

Heater 7 for the reaction tube is operated to heat Ga vapor 5 to 750° C.to perform a process for 10 minutes and subsequently the reaction tubeis slowly cooled again to room temperature. Note that comparativeexamples 5 and 6 are sample substrates which do not undergo the processwith Ga vapor.

The heat treatment with Ga vapor provides graphite film on a surface ofthe CNT network. For each example, the treatment's temperature and theprocessed substrate's sheet resistance value are as shown in table 2.Note that the sheet resistance value is measured in a 4-terminal method.

Furthermore, for each example, on the electrically conductive substratehaving the CNT network having a surface with the graphite film thereon,a resin sheet having thermosetting resin on a surface that is broughtinto contact with the CNT network is deposited from above, and thenthermally set to transfer and secure the CNT network to thethermosetting resin to obtain a transparent, electrically conductivesheet, which has a sheet resistance value as shown in table 2.

TABLE 2 Examples of the Comparative Present Invention Examples 7 8 5 6Amorphous carbon film + − + − Ga vapor process + + − − Ga processtemperature (° C.) 750 750 − − Process time 10 min. 10 min. − −Transparent, electrically  5  10 300 800 conductive sheet's sheetresistance value (kΩ/square)

Examples 9-11 of the Present Invention, and Comparative Examples 7, 8

The FIG. 10 graphite film production apparatus is employed to producegraphite film.

A 1 m long, 25 mm-diameter quartz tube is prepared as quartz reactiontube 6. Quartz reaction tube 6 accommodates an approximately 1cm-diameter alumina container having liquid Ga 9 introduced therein, andin a vicinity thereof, substrate 17 bearing amorphous carbon film 21 isplaced as a substrate to be processed. The substrate to be processed isa silicon substrate having a surface with an approximately 500 nm thickthermal oxide film thereon and an amorphous carbon film provided at thatsurface by laser abrasion.

Examples 9-11 of the Present Invention, and Comparative Example 7

Initially, a substrate to be processed, as aforementioned, is secured inquartz reaction tube 6 horizontally, and a turbo pump is used to vacuumthe background to 10⁻⁶ Torr or lower.

Heater 7 for the reaction tube is operated to heat Ga vapor 5 to atemperature indicated in table 3 to perform a process for one hour, andthe reaction tube is slowly cooled again to room temperature.

Examples 9-11 of the present invention provide approximately 3-5 layersof graphite film on a surface of amorphous carbon film through the heattreatment in Ga vapor. Each sample substrate has a significantly smoothmirror surface without variation in color, or roughness.

Subsequently, the aforementioned amorphous carbon deposition and Gaprocess are repeated until substrate 3 has amorphous carbon film andgraphite film thereon together forming a film having a thickness ofapproximately 50 nm. Each resultant sample substrate has a sheetresistance value, as shown in table 3.

Comparative Example 8

In a comparative example 8, a substrate to be processed which is similarto those of the examples of the present invention is thermally treatedin quartz reaction tube 6 at 600° C. without liquid Ga 1 introducedtherein. In other words, the substrate simply undergoes the heattreatment without subjecting amorphous carbon film to the Ga process.The other steps are performed similarly as done in the above examples ofthe present invention. The resultant sample substrate has a sheetresistance value as shown in table 3.

TABLE 3 Examples of the Comparative Present Invention Examples 9 10 11 78 Process temperature 600 700 800 200 600 (° C.) Sheet resistance value100 20 6 ∞ 1500 (kΩ/square)

Examples 12-14 of the Present Invention, and Comparative Examples 9, 10

The FIG. 11 graphite film production apparatus is employed to producegraphite film.

A 1 m long, 25 mm-diameter quartz tube is prepared as quartz reactiontube 6. Quartz reaction tube 6 accommodates subordinate Ga reactionchamber 22 accommodating an approximately 1 cm-diameter aluminacontainer having liquid Ga 9 introduced therein, and in a vicinitythereof; substrate 17 bearing amorphous carbon film 21 is placed as asubstrate to be processed. The substrate to be processed is a siliconsubstrate having a surface with an approximately 500 nm thick thermaloxide film thereon and an amorphous carbon film provided at that surfaceby laser abrasion.

Examples 12-14 of the Present Invention, and Comparative Example 9

Initially, a substrate to be processed, as aforementioned, is secured insubordinate Ga reaction chamber 22 horizontally, and a turbo pump isused to vacuum the background to 10⁻⁶ Torr or lower.

Heater 7 for the reaction tube is operated to heat Ga vapor 5 insubordinate Ga reaction chamber 22 to a temperature indicated in table 4to perform a process for 10 minutes, and the reaction tube is slowlycooled again to room temperature.

Examples 12-14 of the present invention provide approximately 3-5 layersof graphite film on a surface of amorphous carbon film through the heattreatment in Ga vapor. Each sample substrate has a significantly smoothmirror surface without variation in color, or roughness.

Subsequently, the aforementioned amorphous carbon deposition and Gaprocess are repeated until substrate 17 has amorphous carbon film andgraphite film thereon together forming a film having a thickness ofapproximately 100 nm. For each example, the process's temperature andthe resultant sample substrate's sheet resistance value are as shown intable 4.

Comparative Example 10

In a comparative example 10, a substrate to be processed which issimilar to those of the examples of the present invention is thermallytreated in quartz reaction tube 6 at 600° C. for 10 minutes withoutliquid Ga 1 introduced therein. In other words, the substrate simplyundergoes the heat treatment without subjecting amorphous carbon film tothe Ga process. The other steps are performed similarly as done in theabove examples of the present invention. The resultant sample substratehas a sheet resistance value as shown in table 4.

TABLE 4 Examples of the Comparative Present Invention Examples 12 13 149 10 Process temperature 600 700 800 200 600 (° C.) Sheet resistancevalue 120 30 5 ∞ 2000 (kΩ/square)

Examples 15-17 of the Present Invention, and Comparative Examples 11, 12

The FIG. 12 graphite film production apparatus is employed to producegraphite film.

A 1 m long, 25 mm-diameter quartz tube is prepared as quartz reactiontube 6. Quartz reaction tube 6 accommodates a pair of plasma producingelectrodes 10 therein, and in a vicinity thereof, alumina container 20having a diameter of approximately 1 cm and having liquid Ga 1introduced therein is placed. At the alumina container, heater 12 isplaced for Ga. Substrate 17 bearing amorphous carbon film 21 thereon isplaced between plasma producing electrodes 10 as a substrate to beprocessed. The substrate to be processed is a silicon substrate having asurface with an approximately 500 nm thick thermal oxide film thereonand an amorphous carbon film provided at that surface by laser abrasion.

Examples 15-17 of the Present Invention, and Comparative Example 11

Initially, a substrate to be processed, as aforementioned, is securedbetween plasma producing electrodes 10 horizontally, and a turbo pump isused to vacuum the background to 10⁻⁶ Torr or lower.

Heater 12 for Ga is operated to facilitate evaporating liquid Ga 9,while plasma producing electrodes 10 are used to plasmatize Ga vaporpresent at a location sandwiched between the electrodes, and heater 7for the reaction tube is also used to heat the substrate in contact withGa plasma 23 to a temperature indicated in table 5 and a 10-minuteprocess is also performed, and the reaction tube is slowly cooled againto room temperature.

Examples 15-17 of the present invention each provide approximately 3-5layers of graphite film on a surface of the substrate through the heattreatment in Ga plasma. Each sample substrate has a significantly smoothmirror surface without variation in color, or roughness.

Subsequently, the aforementioned amorphous carbon deposition and Gaprocess are repeated until substrate 17 has amorphous carbon film andgraphite film thereon together forming a film having a thickness ofapproximately 100 nm. For each example, the process's temperature andthe resultant sample substrate's sheet resistance value are as shown intable 5.

Comparative Example 12

In a comparative example 12, a substrate to be processed which issimilar to those of the examples of the present invention is thermallytreated in quartz reaction tube 6 at 600° C. for 10 minutes withoutliquid Ga 9 introduced therein. In other words, the substrate simplyundergoes the heat treatment without subjecting amorphous carbon film tothe Ga process. The other steps are performed similarly as done in theabove examples of the present invention. The resultant sample substratehas a sheet resistance value as shown in table 5.

TABLE 5 Examples of the Comparative Present Invention Examples 15 16 1711 12 Process temperature 400 600 800 200 600 (° C.) Sheet resistancevalue 90 60 3 ∞ 2200 (kΩ/square)

Examples 18-20 of the Present Invention, and Comparative Examples 13, 14

The FIG. 13 graphite film production apparatus is employed to producegraphite film.

A 1 m long, 25 mm-diameter quartz tube is prepared as quartz reactiontube 6. Quartz reaction tube 6 is connected to Ga vapor supply unit 15and hydrocarbon gas supply unit 13. Ga vapor supply unit 15 has liquidGa introduced therein. Hydrocarbon gas supply unit 13 has phenanthreneintroduced therein as a carbon source material. As a substrate to beprocessed, substrate 17 is placed in quartz reaction tube 6.

Examples 18-20 of the Present Invention, and Comparative Example 13

Initially, a substrate to be processed, as aforementioned, is secured inquartz reaction tube 6 horizontally, and a turbo pump is used to vacuumthe background to 10⁻⁶ Torr or lower.

Heater 12 for Ga is used to evaporate liquid Ga 9 to supply Ga vapor tothe interior of quartz reaction tube 6, while valve 16 located betweenhydrocarbon gas supply unit 13 having phenanthrene introduced thereinand quartz reaction tube 6 is opened to supply hydrocarbon gas.

Heater 7 for the reaction tube is operated to raise the temperature inquartz reaction tube 6 to that indicated in table 6 and a 30-minuteprocess is performed, and the reaction tube is slowly cooled again toroom temperature.

Examples 18-20 of the present invention each provide graphite film on asurface of the substrate through the heat treatment in Ga vapor to havea thickness of 200 nm. Each sample substrate has a significantly smoothmirror surface without variation in color, or roughness. For eachexample, the process's temperature and the resultant sample substrate'ssheet resistance value are as shown in table 6.

Comparative Example 14

In a comparative example 14, a substrate to be processed which issimilar to those of the examples of the present invention is thermallytreated in quartz reaction tube 6 at 600° C. for 30 minutes withoutliquid Ga 9 introduced therein. In other words, the substrate simplyundergoes the heat treatment without subjecting amorphous carbon film tothe Ga process. The other steps are performed similarly as done in theabove examples of the present invention. The resultant sample substratehas a sheet resistance value as shown in table 6.

TABLE 6 Examples of the Comparative Present Invention Examples 18 19 2013 14 Process temperature 400 600 800 200 600 (° C.) Sheet resistancevalue 160 40 2 ∞ 1500 (kΩ/square)

INDUSTRIAL APPLICABILITY

The present invention is advantageously applicable particularly tocarbon wires configured of a plurality of short carbon nanotubescombined together, and wire assemblies employing such carbon wires.

Furthermore, the present invention allows mass production of asignificantly thin stack of graphite layers or a monolayer of graphitefilm in large areas. The monolayer of graphite film having a large areacan be used to allow application to an LSI or similar, large scalegraphene integrated circuit. Furthermore, increasing thickness allows atransparent, electrically conductive sheet having a large area to beformed, and it is expected to be applied to a large size liquid crystaldisplay.

DESCRIPTION OF TUE REFERENCE SIGNS

1: carbon wire, 2: carbon nanotube, 3: assembly portion, 4: graphitelayer, 5: wire assembly, 6: quartz reaction tube, 7: heater for reactiontube, 8: evacuation system, 9: liquid Ga, 10: plasma producingelectrode, 11: amorphous carbon layer, 12: heater for Ga, 13:hydrocarbon gas supply unit, 14: reactor, 15: Ga vapor supply unit, 16:valve, 17: substrate, 18: carbon nanotube network, 19: sheet of resin,20: alumina container, 21: amorphous carbon film, 22: subordinate Gareaction chamber, 23: Ga plasma, 24: Ga vapor.

1-13. (canceled)
 14. A method of producing electrically conductive filmhaving a carbon nanotube network formed of a plurality of carbonnanotubes linked together by graphite film, comprising the step ofexposing a carbon nanotube network to Ga (gallium) vapor to provide saidgraphite film.
 15. A method of producing electrically conductive filmhaving a carbon nanotube network formed of a plurality of carbonnanotubes linked together by graphite film, comprising the steps of:providing amorphous carbon film on a carbon nanotube network; andexposing said carbon nanotube network and said amorphous carbon filmobtained in the step of providing, to Ga vapor to provide said graphitefilm.
 16. The method of producing the electrically conductive filmaccording to claim 14, comprising, before the step of exposing, the stepof mechanically pressure-welding those portions of a plurality of carbonnanotubes forming said carbon nanotube network which are in contact withone another.
 17. (canceled)
 18. A method of producing an electricallyconductive substrate formed with a substrate and an electricallyconductive film provided on said substrate and having a carbon nanotubenetwork formed of a plurality of carbon nanotubes linked together bygraphite film, comprising the steps of: forming a carbon nanotubenetwork on a substrate; and exposing said carbon nanotube network to Gavapor to provide said graphite film.
 19. A method of producing anelectrically conductive substrate formed with a substrate and anelectrically conductive film provided on said substrate and having acarbon nanotube network formed of a plurality of carbon nanotubes linkedtogether by graphite film, comprising the steps of: forming a carbonnanotube network on a substrate; providing amorphous carbon film on saidcarbon nanotube network; and exposing said carbon nanotube network andsaid amorphous carbon film that is obtained in the step of providing, toGa vapor to provide said graphite film.
 20. The method of producing theelectrically conductive substrate according to claim 18, comprising,before the step of exposing, the step of mechanically pressure-weldingthose portions of a plurality of carbon nanotubes forming said carbonnanotube network which are in contact with one another.
 21. Atransparent, electrically conductive sheet formed with a sheet of resinand an electrically conductive film provided on said sheet of resin andhaving a carbon nanotube network formed of a plurality of carbonnanotubes linked together by graphite film.
 22. The transparent,electrically conductive sheet according to claim 21, wherein a surfaceof said sheet of resin that has said electrically conductive film isformed of one of thermosetting resin and ultraviolet curable resin. 23.A method of producing the transparent, electrically conductive sheetaccording to claim 21, comprising the steps of: forming a carbonnanotube network on a substrate; exposing said carbon nanotube networkto Ga vapor to provide said graphite film; and transferring to a sheetof resin an electrically conductive film having said carbon nanotubenetwork formed of a plurality of carbon nanotubes linked together bygraphite film in the step of exposing.
 24. A method of producing thetransparent, electrically conductive sheet according to claim 21,comprising the steps of: forming a carbon nanotube network on asubstrate; providing amorphous carbon film on said carbon nanotubenetwork; exposing said carbon nanotube network and said amorphous carbonfilm that is obtained in the step of providing, to Ga vapor to providesaid graphite film; and transferring to a sheet of resin an electricallyconductive film having said carbon nanotube network formed of aplurality of carbon nanotubes linked together by graphite film in thestep of exposing.
 25. The method of producing the transparent,electrically conductive sheet according to claim 23, comprising, beforethe step of exposing, the step of mechanically pressure-welding thoseportions of said plurality of carbon nanotubes forming said carbonnanotube network which are in contact with one another.
 26. The methodof producing the transparent, electrically conductive sheet according toany one of claims 23 and 24 that is a method of producing thetransparent, electrically conductive sheet according to claim 22,wherein the step of transferring transfers said electrically conductivefilm to the surface of said sheet of resin that is formed of one ofthermosetting resin and ultraviolet curable resin, the method furthercomprising the step of setting/curing one of said thermosetting resinand said ultraviolet curable resin.
 27. A method of producing graphitefilm by exposing a surface of a carbon source to Ga vapor to providegraphite film on the surface of said carbon source.
 28. The method ofproducing graphite film according to claim 27, wherein said Ga vapor hasa temperature equal to or higher than 600° C.
 29. The method ofproducing graphite film according to claim 27, wherein said Ga vapor hasa uniform vapor pressure at the surface of said carbon source.
 30. Themethod of producing graphite film according to claim 27, wherein said Gavapor is plasmatized.
 31. The method of producing graphite filmaccording to claim 30, wherein said carbon source is located on asubstrate and said Ga vapor plasmatized is brought into contact withsaid substrate having a temperature equal to or higher than 400° C. 32.The method of producing graphite film according to claim 27, whereinsaid carbon source is amorphous carbon.
 33. The method of producinggraphite film according to claim 32, wherein said amorphous carbon isamorphous carbon film provided on a monocrystalline substrate formed ofone type selected from the group consisting of SiC, Ni, Fe, Mo, and Pt.34. The method of producing graphite film according to claim 27, whereinsaid carbon source is a hydrocarbon material.
 35. The method ofproducing graphite film according to claim 27, wherein said carbonsource is a three dimensional amorphous carbon structure having asurface exposed to Ga vapor to provide graphite film having a threedimensional surface structure.
 36. A method of producing graphite filmby mixing Ga vapor and a source material gas of a carbon source togetherand supplying a mixture thereof to provide graphite film on a substrate.37. The method of producing graphite film according to claim 36, whereinsaid Ga vapor has a temperature equal to or higher than 400° C.
 38. Themethod of producing graphite film according to claim 36, wherein said Gavapor is plasmatized.
 39. The method of producing graphite filmaccording to claim 38, wherein said Ga vapor plasmatized is brought intocontact with said substrate having a temperature equal to or higher than400° C.